Box How many species are there today

So far, 1.7-1.8 million species of plants and animals have been named and described formally (about 270,000 plant species and over 1 million insects). The rate of discovery of new species is highly variable within different groups of organisms. About three new bird species, about one new mammal genus and some 7250 new insect species are named each year.

It might seem easy to use such figures to produce an estimate of global diversity on the assumption that, at some time in the future, all species will have been discovered and named. The simplest approach might be to use a collector curve approach (see p. 165), and document the rate of naming of new species for different groups. This would work well for intensively studied groups, such as birds and mammals, where the analyst would decide that nearly all species have been found. But what of the insects and microbes? These hugely diverse groups are yielding new species as fast as systematists can pin them down and photograph them: their ultimate totals seem limitless. Conservatively, a collector curve of all life might predict that there are 5 million species out there waiting to be discovered.

Other scientists estimate that there are 100 million species on Earth today. They base these figures on a sampling approach. In other words, we can not hope to count or estimate across all of life, but we can take intense samples of certain kinds of organisms and extrapolate from those samples. The best-known example was calculated by entomologist Terry Erwin in the 1980s (e.g. Erwin 1982). Erwin sampled all the beetles from a single species of tropical tree, Luehea seemannii from South America. "Sampled" is a euphemism for "killed" - Erwin set a bug bomb below a tree, and the powerful insecticide knocked everything out and he collected the bodies on a sheet. To his amazement, Erwin found dozens of new species in each sample. He estimated that this one tropical tree species carried 1100 beetle species, of which about 160 were unique to each tree species. There are about 50,000 tropical tree species around the world, and if the numbers of endemic beetle species in L. seemanni is typical, this implies a total of 8.15 million canopy-dwelling tropical beetle species in all (50,000 x 160). Beetles typically represent about 40% of all arthropod species, and this leads to an estimate of about 20 million tropical canopy-living arthropod species. In tropical areas, there are typically twice as many arthropods in the canopy as on the ground, giving an estimate of 30 million species of tropical arthropods worldwide. This estimate came as a considerable surprise when it was published: 30 million species of tropical arthropods must imply a global diversity of all life in the region of 50 million. Some wild-eyed biologists even talked of figures of 100 million or more!

Subsequent authors have pointed out that Luehea was uniquely rich in its own special beetle species, and the global estimate should be nearer 15-20 million species than 50-100 million. The debate continues. It is worth noting that if is so difficult to estimate modern biodiversity, what hope do paleontologists have of providing accurate estimates for total biodiversity in the past?

marine invertebrates had been a rapid rise to modern diversity levels during the Cambrian and Ordovician, and a steady equilibrium level since then (Fig. 20.1b).

The to-and-fro debate about the reality of such broad-scale diversification patterns has continued since the early 1970s, with many proposals that the pattern is broadly correct (our view), but with many current and serious challenges (Smith 2007) (see pp. 72-7). Many diversification plots show similar patterns, with slow rates of diversification at first, many set-backs, and a rapid rate of increase over the past 100 myr, with no sign of a leveling off. This is true of vertebrates, insects and plants, and the latest plots for marine animals are comparable with Valentine's original plot (Fig. 20.2). If the diversification curves show something about evolution, how should they be interpreted?